Hello, I got given a G300 for xmas- it is sim card unlocked. When I put my O2 sim card in the phone will reach the home screen and after 5 seconds it will restart. It will carry this process on and on. However, when I try my spare T-mobile sim card, the phone will turn on and stay on...My O2 sim card seems to be working fine as I am currently using it. Any idea what the problem could be regarding this issue? Thanks for your help, Michael.